1. Field of the Invention
The present invention relates to a power transmission mechanism mounted in a hybrid vehicle or the like whose travel is powered by using, for example, an internal combustion engine and an electric motor together, and the driving force of at least one of the internal combustion engine or the electric motor is transmitted to the driving wheels of the vehicle.
2. Description of the Related Art
The conventional power transmission mechanism 1 is known such as that shown in FIG. 7 that comprises an internal combustion engine 2, an electric motor 3, an input shaft 4, an output shaft 5, a main clutch 6, a transmission 7, a driving force transmission switching unit 8, and an ECU 9, and the driving force of at least one of the internal combustion engine 2 or the electric motor 3 is transmitted to the drive wheels W and W of the vehicle.
In this power transmission 1, the internal combustion engine 2 communicates with the input shaft 4 of the transmission 7 via the main clutch 6, and the rotation shaft 3a of the electric motor 3 communicates with the output shaft 5 of the transmission 7 via the driving force transmission switching unit 8.
The transmission 7 is an automatic type transmission in which, for example, the transmission operation is controlled by a plurality of gear actuators (not illustrated) being actuated by the ECU 9, and selecting one of a plurality of gear sets (not illustrated) having different gear ratios.
In addition, the driving force transmission switch 8 comprises a switching actuator 8a controlled by the ECU 9 and a switching synchronous clutch 8b actuated by the switching actuator 8a, and, and this forms a switching mechanism that engages or disengages either one of the rotation shaft 3a of the electric motor 3 or the input shaft 4 and the output shaft 5 of the transmission 7 via the switching synchronous clutch 8b. 
For example, during start of the internal combustion engine 2 when the vehicle is in a stationary state, the driving force transmission switching unit 8 engages the rotation shaft 3a of the electric motor 3 and the input shaft 4 of the transmission 7, the driving force of the electric motor 3 is transmitted to the internal combustion engine 2, and thereby the internal combustion engine 2 starts. Then, for example, in the EV travel mode in which the vehicle is actuated by the driving force of the electric motor 3, the driving force transmission switching unit 8 engages the rotation shaft 3a of the electric motor 3 and the output shaft 5 of the transmission 7, the driving force of the electric motor 3 is transmitted to the drive wheels W and W, and thereby the vehicle is actuated.
Moreover, in the EV travel mode, for example, the internal combustion engine 2 is stopped, and while the internal combustion engine 2 and the transmission 7 are disengaged by the main clutch 6, the vehicle is actuated by the electric motor 3.
In the case that a start request for the internal combustion engine 2 is generated in the EV travel mode, based on the current velocity of the vehicle and the selectable gear ratios in the transmission 7, the ECU 9 carries out transmission operation in the transmission by actuating the appropriate transmission actuators such that the rotation speed (the rotation speed of the input shaft) NI of the input shaft 4 has a rotation speed equal to or greater than the predetermined start allowing rotation speed #N0 that is the minimum rotation speed for the start of the internal combustion engine 2, and thereby the input shaft 4 and the output shaft 5 are engaged through the transmission 7.
In addition, the ECU 9 starts the internal combustion engine 2 equal to or greater than a predetermined start allowing rotation speed #N0 by actuating the switching actuator 8a of the driving force transmission switching unit 8, engaging the rotation shaft 3a of the electric motor 3 to the output shaft 5 of the transmission 7, actuating the clutch actuator 6a, and engaging the internal combustion engine 2 and the transmission 7 via the main clutch 6.
However, in the power transmission 1 according to the example of the conventional technology described above, as shown, for example, in FIG. 8, the change of the input shaft rotation speed NI that depends on the velocity V of the vehicle (vehicle speed) is set for each of the selectable gear ratios in the transmission 7. Here, for example, in the case that the vehicle speed V is in a very low speed state near zero (for example, less than the minimum velocity V0 shown in FIG. 8), even in the case that any selectable gear ratio in the transmission 7 is selected, the input shaft rotation speed NI cannot be set equal to or greater than the predetermined start allowing rotation speed #N0, and thus there are cases in which the internal combustion engine 2 cannot be started up.
For this problem, in the EV travel mode, a method is known, for example, in which the input shaft 4 and the output shaft 5 are disengaged by setting the transmission 7 to neutral, the rotation shaft 3a of the electric motor 3 and the output shaft 5 are disengaged by the driving force transmission switching unit 8, the rotation shaft 3a and the input shaft 4 are disengaged, and thereby the driving force of the electric motor 3 is transmitted to the internal combustion engine 2, and the internal combustion engine 2 is started up equal to or greater than a predetermined start allowing rotation speed #N0. However, in this case, during the time interval until the start of the internal combustion engine 2 has completed, because the driving force for actuating the vehicle is zero, the vehicle cannot be actuated in response to the requested output from the driver, and there is the problem that the driver is made to feel discomfort towards the behavior of the vehicle.
In consideration of the problems described above, it is an object of the present invention to provide a power transmission mechanism that can reliably start the internal combustion engine of a vehicle that is stopped during travel.